1. Field of the Invention
The invention relates to a process for the determination of small ion concentrations in aqueous specimens by ion chromatography with an aqueous eluent, and also to an apparatus for the execution of the process.
2. Description of the Prior Art
Aqueous specimens, specifically those which are obtained from the environment, such as atmospheric condensates, bored ice cores from glaciers, or hailstones, contain trace amounts of ionic impurities, the measurement of which is extraordinarily important for a determination of current conditions and changes relative to previous measurements.
In terms of order of magnitude, such trace impurities may lie in the range of ppb (parts per billion, or 1.times.10.sup.-9) so that the sensitivity of the analysis equipment must be high. Commercial devices generally have their detection limits in the range of 100 ppb.
FIG. 6 shows the curve of the conductivity of fluid exiting the separation column of the chromatograph in a conventional method of operation. It can be seen that, at the beginning of the chromatogram, there is a severe drop in conductivity. This is caused by the low conductivity of the water of the specimen, which is normally added into the eluent current.
According to the invention, this decrease in conductivity is prevented as a result of the fact that a stream of concentrated eluent is conducted to the column, which is diluted to the desired concentration prior to introduction into the column with a stream of extra-pure water into which the specimen is introduced prior to mixing. In this manner, the water of the specimen itself presents no further interference during the following chromatographic process. Extra-pure water is water which has been freed of impurities as far as possible, and whose level of impurities will not interfere with the intended detection, that is, water whose concentration of impurities can no longer be detected by the measurement apparatus. This type of water for analysis is produced in a known manner by multiple distillation and/or ion exchange processes, and may be obtained from the Fa. Millipore as "Milli-Q", especially type 1, having an electrical resistivity of 16.66 meg-ohm per cm. The chromatogram illustrated in FIG. 6 for anions can, of course, also be taken with an appropriate column packing and elution agent for cations.